Defining Florida Bay

by Christine Rapozo

University of Florida/Monroe County Cooperative Extension

Florida Bay Education Project

Is Florida Bay an Estuary or Tropical Lagoon, and Why does it Matter?

Understanding and resolving the problems facing Florida Bay first requires a definition of what Florida Bay is. This definition was a topic of discussion at a seagrass research workshop in Key Largo. Scientists debated whether Florida Bay is an Estuary or a Tropical Lagoon. The difference in definition could have a considerable influence on how the upstream water supply and resultant salinity in the Bay is managed.

What is an Estuary?

An estuary is a semi-enclosed body of water where fresh water from the land, usually from a river, meets salt water from the sea . As the waters meet and mix, the salt content of the water gradually goes from zero parts of salt per thousand (ppt) at the rivers head to full strength sea water (usually 32-38ppt ) at the ocean inlet. The area in between, called the mixing zone, has a salt content that varies with tide, season, depth and distance from the sea or river.

What is a Tropical Lagoon?

A tropical lagoon is bordered by land on at least one side and is blocked from the sea by sandbars or coral reefs. Lagoons are typically shallow and do not have a major point source of fresh water input, such as a river, but do collect fresh water as it is discharged from the land through storm water runoff. The salinity of a lagoon depends on; seasonal rainfall, fresh water flowing from the land, evaporation, and the exposure to sea water. Salinity is usually lower near the land and greater near the sea. Because lagoons are shallow, surface and bottom waters often mix so the salinity is the same at different depths. Some lagoons may also have areas of hypersalinity, where the salt content is higher than normal sea water, due to evaporation.

So, salinity is the difference?

Yes, what it really comes down to between an estuary or tropical lagoon, is the difference in the salinity profile. Influences from the land, and the mixing of fresh and salt water, create unique habitats. It is these habitats that make the areas so productive. Many marine organisms rely on having just the right salinity at just the right time to ensure survival of their young. The ability to physically tolerate a range of salinity allows some organisms to live and compete for resources where others cannot. That is why estuaries are commonly referred to as nursery grounds because juvenile fish, shrimp, crab, lobster, and crocodile prefer lower salinity environments. Tropical Lagoons can be as productive as estuaries however, the higher salinities limit the variety of species that can live there.

What does Florida Bay look like?

Florida Bay fills an 850 square mile triangle between the fresh water Everglades, the Florida Keys, and the Gulf of Mexico. It is riddled with shallow basins, miles of mud banks, and mangrove covered keys. Florida Bay receives fresh water through two Everglades' sloughs (slow flowing, seasonally charged, broad, shallow waterways) and a man-made canal system. The C-111 basin is a man-made flood protection and water control system which releases fresh water throughout the year from Trout Creek to Highway Creek in northeast Florida Bay. Fresh water from Taylor Slough enters north central Florida Bay between Little Madiera Bay and Rankin Bight. Shark River Slough does not enter the Bay directly, but empties into the Gulf of Mexico where currents push its fresh water into the western edge of the Bay. Saltwater enters the Bay from the Gulf of Mexico, with small inputs from the Atlantic Ocean. As a result the factors determining salinity in Florida Bay are very complicated. Historically, salinity was influenced by location, wind, season, tide, long term climatic cycles, storms and hurricanes. With the presence of man in south Florida, the Bay's salinity is now also affected by water management for flood protection, urban development and agriculture. In a very general sense, northeast Florida Bay is more fresh, with salinities from zero to low 20's ppt. Salinities in the central and western parts of the Bay range between 20-35 ppt. However, during drought years, central and western Florida Bay have had salinities exceeding 70 ppt (that's twice the salt content of sea water). This hypersalinity occurs when evaporation exceeds fresh water inputs, concentrating the salt.

Is Florida Bay an Estuary or Lagoon?

This is not an easily answered question. Florida Bay has features of both a tropical lagoon and an estuary. Today, with the man made modifications, eastern Florida Bay often acts more as an estuary while central and western Florida Bay are more like a tropical lagoon.

Why does it matter?

From a resource management perspective for seagrasses, controlling Florida Bay's salinity creates a dilemma. All that is needed to make the Bay more estuarine is to increase fresh water flow through the Everglades. Salinities would be lowered leading to a diverse environment of shoal grass, manatee grass and turtle-grass beds. However, if resource management focuses on recreating the Bay's once lush turtle-grass beds present prior to seagrass dieoff, then a tropical lagoon environment with higher salinity would be required.

For more information contact the Florida Bay Education Project at 305-852-3592. Additional information on a variety of topics is available from the University of Florida/Monroe County Cooperative Extension Service, 5100 College Road, Stock Island or call at 292-4501; fax: 292-4415; email: monroe@mail.ifas.ufl.edu or visit our web site http://monroe.ifas.ufl.edu. Our services are free and available to all without regard to race, color, sex, or national origin.